FIGS. 11(a)-11(k) are perspective views illustrating process steps in a method of fabricating an integrated semiconductor light modulator and laser according to the prior art.
Initially, as illustrated in FIG. 11(a), a pair of dielectric films 2a and 2b comprising SiO.sub.2 or the like are formed on prescribed regions of an n type InP substrate 1 with an unmasked region 1a of the substrate i between them. That is, the surface of the n type InP substrate i is divided into regions in the longitudinal direction, and the dielectric films 2a and 2b are formed on two regions where a semiconductor laser is to be fabricated (hereinafter referred to as laser region A). A laser oscillating part of the semiconductor laser will be produced on the unmasked region 1a between the dielectric films 2a and 2b.
FIG. 10(a) is a plan view illustrating a semiconductor wafer 100 including a plurality of chip regions each corresponding to the semiconductor chip 201 shown in FIG. 11(a). FIG. 10(b) is an enlarged view of a part of the wafer 100. As shown in FIGS. 10(a) and 10(b), the dielectric films 2a and 2b are formed on the wafer 100 so that the respective chip regions 201 have the dielectric films 2a and 2b in the same position.
In the step of FIG. 11(b), using the dielectric films 2a and 2b as masks, the surface of the n type InP substrate 1 is etched to a depth of 1.0.about.1.5 .mu.m.
In the step of FIG. 11(c), using the dielectric films 2a and 2b as masks, an active layer 4 is selectively grown on the surface 1c of the n type InP substrate 1 exposed in the above-described etching process. More specifically, the active layer 4 comprises an n type InP cladding layer about 0.3.about.0.5 .mu.m thick, an undoped-InGaAsP/InGaAs MQW (multiquantum well) layer about 0.1 .mu.m thick, and a p type InP cladding layer about 0.3.about.0.5 .mu.m thick. These layers are successively grown by MOCVD (Metal Organic Chemical Vapor Deposition). The undoped InGaAsP/InGaAs MQW layer serves as a light absorption layer in a light modulator and as an optical waveguide layer in a region between the semiconductor laser and the light modulator.
After removal of the dielectric films 2a and 2b, a photoresist film is deposited on a portion of the active layer 4 grown on the region 1a of the n type InP substrate 1 and on the surface of the n type InP substrate 1 exposed by the removal of the dielectric films 2a and 2b. Using the photoresist film as a mask, a diffraction grating 5 is produced in the laser region A by photolithography and etching techniques as shown in FIG. 11(d). Preferably, an interference exposure technique is employed for the photolithography. The diffraction grating 5 is produced to make a distributed feedback (DFB) semiconductor laser.
Thereafter, as shown in FIG. 11(e), a p type InP layer 6 is grown over the entire surface of the substrate 1 by MOCVD so that the diffraction grating 5 is completely embedded in the InP layer 6.
In the step of FIG. 11(f), the p type InP layer 6, the active layer 4, and the n type InP substrate 1 are selectively etched with HBr etchant to form a stripe-shaped mesa 7 having a height of about 4 .mu.m and a width of 2.about.3 .mu.m.
In the step of FIG. 11(g), an Fe-doped high resistivity InP layer 8 about 3 .mu.m thick and an n type InP hole trap layer 9 about 1 .mu.m thick are successively grown on the substrate 1, contacting the opposite sides of the stripe-shaped mesa 7. Preferably, these layers are grown by MOCVD.
In the step of FIG. 11(h), a p type InGaAsP contact layer 10 about 0.2 .mu.m thick is grown on the mesa 7 and on the InP hole trap layer 9, preferably by MOCVD.
Thereafter, as shown in FIG. 11(i), the p type InGaAsP contact layer 10 is selectively etched away, leaving a portion 10a in the laser region A and a portion 10b on the p type InP layer 6 in a region where a light modulator is to be produced (hereinafter referred to as modulator region B).
In the step of FIG. 11(j), a surface protection film 11 comprising SiO.sub.2 or the like is selectively formed on the n type InP hole trap layer 9 exposed by the selective removal of the contact layer 10 and on the peripheral portions of the remaining contact layers 10a and 10b. Preferably, this surface protection film 11 is formed by thermal CVD.
Thereafter, a p side electrode 12 for the light modulator and a p side electrode for the semiconductor laser are formed in contact with the p type InGaAsP contact layers 10b and 10a, respectively. Preferably, these p side electrodes 12 and 13 comprise Ti/Pt/Au. Further, an n side electrode 14 comprising, for example, AuSn/Au, is formed on the rear surface of the n type InP substrate 1. Further, an anti-reflection coating (not shown) is formed on a facet of the light modulator from which laser light is emitted. Thus, an integrated semiconductor light modulator and laser 200 in which a semiconductor laser 200a and a light modulator 200b are integrated on the same substrate 1 is completed (FIG. 11(k)).
Generally, when a III-V compound semiconductor layer is grown on a substrate with a dielectric mask pattern by MOCVD, the composition of the grown semiconductor layer varies due to the phenomenon described in the following, whereby the photoluminescence (PL) peak wavelength, i.e., the band gap energy, of the semiconductor layer varies. That is, an organometallic compound that contains group-III atoms, such as TEG (triethylgallium) or TMI (trimethylindium), is applied to the surface of the semiconductor substrate and thermally decomposed at the surface of the substrate, producing group-III atoms. These group-III atoms are taken in appropriate positions of the crystal lattice of the semiconductor substrate. Meanwhile, group-V atoms are produced by thermal decomposition of AsH.sub.3 (arsine) gas or PH.sub.3 (phosphine) gas. These group-III atoms and group-V atoms combine with each other at the lattice positions, whereby a crystal layer is grown. On the other hand, the group-III atom containing organometallic compound applied to the dielectric mask is not thermally decomposed but moves on the surface of the dielectric mask. This phenomenon is called "migration". As the result of the migration, the organometallic compound is taken in the crystal layer growing on the semiconductor substrate in the vicinity of the dielectric mask, and the group-III atoms included in the organometallic compound are combined with the group-V atoms as described above. Therefore, the crystal layer grown on a region of the substrate in the vicinity of the dielectric mask has a group-III element composition ratio larger than that of the crystal layer grown on other regions. Therefore, the PL peak wavelength, i.e., the band gap energy, of the semiconductor layer grown on the region in the vicinity of the dielectric film shifts toward the longer wavelength side, i.e., the smaller energy side, compared to that of the semiconductor layer grown on the region far from the dielectric mask. Further, the growth rate of the semiconductor layer increases in the region near the dielectric mask because of the above-described migration of the group-III atoms and, therefore, the grown semiconductor layer is thicker in the region near the dielectric mask than in other regions. For example, when an MQW layer is grown on the semiconductor substrate with the dielectric mask, the thickness of the well layer included in the MQW layer varies, whereby the PL peak wavelength of the MQW layer grown on the region in the vicinity of the dielectric mask shifts toward the long wavelength side. In this way, when a semiconductor layer is grown on the semiconductor substrate partially masked with the dielectric film, the PL peak wavelength of the semiconductor layer grown on the semiconductor substrate is varied.
According to the above-described principle, in the fabrication process of the integrated semiconductor light modulator and laser chip 200, the group-III element composition ratio and the thickness of the active layer 4 including the MQW layer is larger in the region 1a of the substrate 1 sandwiched between the dielectric masks 2a and 2b than in the modulator region B where the dielectric mask is absent because a lot of group-III atoms migrate from the dielectric masks 2a and 2b to that region 1a. Therefore, the PL peak wavelength of the active layer 4 in the laser oscillation region 1a of the laser region A shifts toward the longer wavelength side from the PL peak wavelength of the active layer 4 in the modulator region B.
Furthermore, when the width of the unmasked region 1a between the dielectric masks 2a and 2b is varied by varying the widths of the dielectric masks 2a and 2b, the quantity of the group-III atoms migrating from the dielectric masks 2a and 2b into the growing semiconductor layer on the region 1a changes, whereby the composition, i.e., the PL peak wavelength, of the grown semiconductor layer changes. As the width of the region 1a is reduced by increasing the widths of the dielectric masks 2a and 2b, the PL peak wavelength shifts toward the longer wavelength side. On the other hand, as the width of the region 1a is increased by decreasing the widths of the dielectric masks 2a and 2b, the PL peak wavelength shifts toward the shorter wavelength side.
In this way, in the integrated semiconductor light modulator and laser chip 200, the PL peak wavelength of the MQW light absorption layer of the light modulator 200b is smaller than the PL peak wavelength of the MQW active layer of the semiconductor laser 200a. Therefore, when a forward producing a current exceeding a threshold current is applied to the semiconductor laser 200a, laser light emitted from the semiconductor laser 200a travels through the MQW optical waveguide layer interposed between the active layer of the semiconductor laser 200a and the light absorption layer of the modulator 200b and through the light absorption layer of the modulator 200b and is output from the facet of the modulator 200b. During the oscillation of the semiconductor laser 200a, when a reverse bias is applied to the light modulator 200b, the absorption end of the light absorption layer shifts toward the long wavelength side, and the light absorption layer absorbs the laser light. Because of this principle, the light modulator 200b is called an electroabsorption modulator. Therefore, when the bias voltage applied to the light modulator 200b is controlled while causing oscillating the semiconductor laser 200a, the intensity of the laser light emitted from the facet of the modulator 200b is modulated, i.e., on and off switching of the laser light is performed. In this way, the light modulator 200b is used as a light source for optical communication.
In the above-described fabrication method, since the semiconductor laser 200a and the light modulator 200b are simultaneously produced on the same semiconductor substrate 1, the fabrication process is simplified. In addition, since the semiconductor layers included in the semiconductor laser 200a are crystallographically continuous with the semiconductor layers included in the light modulator 200b, the transmission .loss of the laser light traveling through the region between the laser 200a and the modulator 200b is reduced. However, in order to obtain optical signals with higher stability, a lot of problems remain to be solved. These problems will be described in detail hereinafter.
FIG. 12(a) is a plan view illustrating a chip region 201 with a pair of dielectric masks 2a and 2b, corresponding to the step shown in FIG. 11(a). FIG. 12(b) is a graph illustrating variations in the crystal composition of an InGaAsP/InGaAs MQW layer selectively grown on the chip region 201 with the dielectric masks 2a and 2b. The variations in the crystal composition are shown by PL peak wavelengths of the MQW layer measured along the optical waveguide direction of the MQW layer. In FIG. 12(a), reference numeral 1a designates a region of the MQW layer between the dielectric masks 2a and 2b where a semiconductor laser is to be fabricated (hereinafter referred to as laser region), and numeral 1b designates a region of the MQW layer where a light modulator is to be fabricated (hereinafter referred to as modulator region). In FIG. 12(b), the ordinate shows PL peak wavelengths of the MQW layer, and the abscissa shows positions on the chip region along what becomes the resonator length direction of the semiconductor laser. As shown in FIG. 12(b), the crystal composition of the MQW layer grown on the laser region 1a between the dielectric masks 2a and 2b is not stable in the vicinity of the boundary between the laser region 1a and the modulator region 1b. The crystal composition becomes stable at a distance from the boundary, and the PL peak wavelength at this portion is the emission wavelength of the semiconductor laser.
Further, the crystal composition of the MQW layer grown on the modulator region 1b where no dielectric mask is present is not stable in the vicinity of the boundary between the laser region 1a and the modulator region 1b. The crystal composition becomes stable at a distance from the boundary, and the PL peak wavelength at this portion is the absorption wavelength of the light modulator. Hereinafter, a portion of the MQW layer at the boundary between the laser region 1a and the modulator region 1b is called a transition region where the crystal composition, i.e., the PL peak wavelength, of the MQW layer gradually changes.
The performance of the device is affected by the length (L) of the transition region. FIG. 13 is a graph illustrating variations in the crystal composition of a modulator integrated semiconductor laser device which is produced so that the absorption wavelength .lambda..sub.MOD of the light modulator is 1.50 .mu.m and the emission wavelength .lambda..sub.LD of the semiconductor laser is 1.55 .mu.m. The variations in the crystal composition of the laser device are shown by PL peak wavelengths measured along the optical waveguide direction. In FIG. 13, it is assumed that the transition region between the semiconductor laser and the light modulator has a crystal composition corresponding to a wavelength .lambda..sub.trans. of 1.525 .mu.m. It is well known that a crystal layer having a crystal composition corresponding to a wavelength of 1.525 .mu.m serves as an absorber for light having a wavelength of 1.55 .mu.m, and the absorption coefficient .alpha. is about 780 cm.sup.-1. In the transition region shown in FIG. 13, if it is assumed that the coefficient of optical confinement of the optical waveguide layer to the cladding layer is 10%, the substantial absorption coefficient .alpha.' is 78 cm.sup.-1. The light intensity I, which is obtained when light having an intensity I.sub.0 has passed through an absorber having an absorption coefficient .alpha. and a thickness L, is represented by EQU I=I.sub.0 exp(-.alpha.L)
Therefore, the ratio R of the intensity I of the light that has passed through the absorber to the intensity I.sub.0 of the light before passing through the absorber is represented by EQU R=I/I.sub.0 =exp(-.alpha.L)
Assuming that the length L of the transition region is 200 .mu.m, the light intensity ratio R is calculated as follows: ##EQU1##
Assuming that the length L of the transition region is 50 .mu.m, the light intensity ratio R is calculated as follows: ##EQU2##
That is, when the length L of the transition region decreases to 1/4, i.e., from 200 .mu.m to 50 .mu.m, the intensity of laser light traveling from the semiconductor laser to the light modulator increases by three times, i.e., from 21% to 68%. Therefore, as the length L of the transition region decreases, the attenuation in the intensity of the laser light before reaching the light modulator is suppressed, whereby a stable optical signal with a high on/off ratio is obtained.
However, in the prior art method of fabricating a modulator integrated semiconductor laser device described with respect to FIGS. 11(a)-11(k), the width of the dielectric masks 2a and 2b and the width of the opening 1a between the dielectric masks 2a and 2b are predetermined to attain a desired emission wavelength of the semiconductor laser. In this case, however, since the length L of the transition region is unconditionally determined, it is not possible to reduce the length of the transition region.
Recently, an improved method of fabricating a modulator integrated semiconductor laser device in which a region where laser light is substantially absorbed is reduced has been developed.
FIGS. 14(a) and 14(b) are a perspective view and a plan view for explaining the improved method of fabricating a modulator integrated semiconductor laser device. In these figures, a pair of dielectric masks 22a and 22b having an opening 1a are disposed on a region A of the surface of the n type InP substrate 1 where a semiconductor laser is to be fabricated, and a pair of dielectric masks 23a and 23b having an opening 1b are disposed on a region C of the surface of the n type InP substrate where a light modulator is to be produced. Preferably, these dielectric masks 22a, 22b, 23a, and 23b comprise SiO.sub.2. The width w.sub.4 of the unmasked portion 1b between the dielectric masks 23a and 23b is larger than the width w.sub.3 of the unmasked portion 1a between the dielectric masks 22a and 22b.
The fabrication process of this modulator integrated semiconductor laser is basically identical to the process already described with respect to FIGS. 11(a)-11(k) except that the dielectric mask pattern shown in FIG. 14(a) is formed in the step of FIG. 11(a). The emission wavelength of the semiconductor laser is controlled by the width w.sub.3 of the unmasked portion 1a between the dielectric masks 22a and 22b, and the absorption wavelength of the light modulator is controlled by the width w.sub.4 of the unmasked portion 1b between the dielectric masks 23a and 23b.
In this fabrication process, the length of the laser light absorbing portion in the transition region between the laser region and the modulator region can be reduced. The reason will be described hereinafter.
FIG. 14(c) is a graph illustrating variations in the crystal composition of an InGaAsP/InGaAs MQW layer that is grown on the n type InP substrate 1 with the dielectric mask pattern shown in FIG. 14(b) by MOCVD. The variations in the crystal composition are shown by PL peak wavelengths of the MQW layer measured along the optical waveguide direction. In FIG. 14(c), a curve c shows variations in the crystal composition (PL peak wavelength) of the MQW layer due to the dielectric masks 22a and 22b, and a curve d shows variations in the crystal composition (PL peak wavelength) of the MQW layer due to the dielectric masks 23a and 23b. A broken line shows variations in the crystal composition (PL peak wavelength) of the MQW layer due to the dielectric masks 22a, 22b, 23a, and 23b.
As shown in FIG. 14(c), the length L of the transition region where the PL peak wavelength of the MQW layer is not stable is longer than that of the laser device fabricated by the method shown in FIGS. 11(a)-11(k). However, the length L.sub.A of a part of the transition region, which part has a wavelength longer than the absorption wavelength .lambda..sub.MOD of the modulator region and absorbs laser light having the emission wavelength .lambda..sub.LD of the semiconductor laser, is about half of that of the laser device fabricated by the method of FIGS. 11(a)-11(k) in which the entire transition region has a wavelength longer than the absorption wavelength .lambda..sub.MOD and absorbs the laser light. A part of the entire transition region having a length L.sub.B has a wavelength shorter than the absorption wavelength .lambda..sub.MOD of the modulator, so that this part does not absorb the laser light having the laser wavelength .lambda..sub.LD. Therefore, a pair of dielectric masks 22a and 22b and a pair of dielectric masks 23a and 23b are disposed in the laser region and in the modulator region, respectively, so that the width w.sub.4 of the opening 1b between the dielectric masks 23a and 23b is larger than the width w.sub.3 of the opening 1a between the dielectric masks 22a and 22b, whereby a portion having a PL peak wavelength shorter than the PL peak wavelength (emission wavelength) of the active layer of the semiconductor laser and the PL peak wavelength (absorption wavelength) of the light absorption layer of the light modulator is produced in the optical waveguide layer connecting the active layer and the light absorption layer. Therefore, the length of the portion of the optical waveguide layer which substantially absorbs laser light produced in the semiconductor laser is significantly reduced compared to the laser device fabricated with the dielectric mask pattern shown in FIG. 12.
FIG. 15(a) is a perspective view illustrating a modification of the dielectric mask pattern shown in FIG. 14(a). In the figure, the same reference numerals as in FIG. 14(a) designate the same or corresponding parts.
In the dielectric mask pattern shown in FIG. 14(a), the width w.sub.4 of the opening 1b between the dielectric masks 23a and 23b is larger than the width w.sub.3 of the opening 1a between the dielectric films 22a and 22b. However, in the dielectric mask pattern shown in FIG. 15(a), the width w.sub.4 is equal to the width w.sub.3. In this case, the profile of the crystal composition (PL peak wavelength) as shown in FIG. 14(c) is achieved by appropriately selecting the width of the dielectric film 22a (22b) and the width of the dielectric film 23a (23b). Also in this case, the same effects as described above are achieved.
FIG. 15(b) is a perspective view illustrating another modification of the dielectric mask pattern shown in FIG. 14(a). In the figure, the same reference numerals as in FIG. 14(a) designate the same or corresponding parts. Japanese Published Patent Application No. Hei. 5-82909, discloses a method of producing a semiconductor layer including a laser region and a light modulator region using a mask for selective growth having the same pattern as that shown in FIG. 15(b).
The dielectric mask pattern shown in FIG. 15(b) includes a pair of dielectric masks 24a and 24b on a region D between the laser region A and the modulator region B, in addition to the mask pattern shown in FIG. 15(a). The width of the dielectric mask 24a (24b) is smaller than the width of the dielectric mask 23a (23b). The width w.sub.5 of an unmasked portion of the substrate 1 between the dielectric masks 24a and 24b is selected so that the relationship of w.sub.3 .ltoreq. w.sub.5 .ltoreq.w.sub.4 is satisfied.
In this case, by appropriately selecting the width of the dielectric mask 24a (24b) and the width w.sub.5 of the opening between the dielectric masks 24a and 24b, the profile of the crystal composition of the optical waveguide layer (transition region) connecting the active layer of the laser and the light absorption layer of the modulator is improved. For example, a steep change of the crystal composition is achieved at the boundary between the active layer and the optical waveguide layer. As the result, the performance of the integrated semiconductor light modulator and laser device is significantly improved.
In the prior art methods of fabricating integrated semiconductor light modulator and laser devices described with respect to FIGS. 14(a)-14(c) and 15(a)-15(b), although the laser light absorbing portion in the transition region is reduced, the length of the transition region itself is increased, resulting in an increase in the chip size of the device.
Furthermore, in the above-described prior art methods of fabricating integrated semiconductor light modulator and lasers, since the dielectric mask pattern is produced on the semiconductor wafer with a pair of dielectric masks disposed in the same position on each chip region, the semiconductor laser section of the chip region 201 is in contact with the light modulator section of an adjacent chip region in the longitudinal direction of the chip region, as shown in FIG. 10(b). Therefore, as shown in FIG. 10(c), the thickness of the optical waveguide region of the semiconductor layer unfavorably varies at the edges of the laser region and the modulator region in contact with the adjacent chip region, resulting in unstable wavelength light by emission the laser and unstable wavelength absorption by the modulator.